In the third volume of the classic children’s mystery series, the Hardy Boys’ criminal investigation at an old mill puts the teens in danger.   Someone is passing counterfeit bills around their hometown, and Frank and Joe Hardy are convinced it has something to do with the mysterious happenings at the Old Mill. But as soon as they start investigating, someone tampers with their speedboat, and the brothers are nearly killed! With their father away on a confidential case, Frank and Joe are determined to uncover the mysteries of the Old Mill, even as danger looms . . .